A secondary battery is a battery which is able to convert chemical energy to electrical energy and discharge. It is also a battery that is able to convert electrical energy to chemical energy and store the chemical energy (charge) by passing a current in a direction that is opposite to the direction upon discharge.
In recent years, secondary batteries such as a lithium secondary battery are widely used as in-vehicle power sources or power sources for portable devices such as notebook PCs and mobile phones. Especially, an all-solid-state secondary battery comprising a solid electrolyte, instead of a liquid electrolyte using a combustible organic solvent as the solvent, has attracted attention as a battery which is safe from liquid leakage and excellent in stability.
A secondary battery generally comprises: a cathode having a cathode active material layer which contains a cathode active material; an anode having an anode active material layer which contains an anode active material; and an electrolyte layer disposed between the electrodes.
To improve the performance of secondary batteries, considerable research has been conducted on cathode active materials and electrolytes for use in secondary batteries (for example, see Patent Literatures 1 to 3).
For example, in Patent Literature 1, a cathode production method is used as a method for producing a cathode for secondary batteries, in which a cathode active material, an electroconductive material and a binder are mixed in the presence of a solvent to obtain a slurry, and the slurry is applied onto a current collector, dried and then pressurized, thereby producing a cathode. Also in Patent Literature 1, an oil absorption amount is defined as an indicator of the coating property of the cathode active material, on the premise of slurrying the cathode active material, and it is mentioned that a secondary battery in which the cathode uses the cathode active material having an oil absorption amount of 30 ml or more and 50 ml or less per 100 g, with respect to N-methylpyrrolidone (NMP), has excellent battery characteristics.
In the case of a secondary battery in which a liquid electrolyte is used in the electrolyte layer, the liquid electrolyte penetrates into the space between the particles of the cathode active material in the cathode active material layer. Therefore, the interface area between the cathode active material and the electrolyte is increased and makes it possible to efficiently conduct ions.
On the contrary, in the case of an all-solid-state secondary battery, since the electrolyte layer is a solid electrolyte layer comprising a solid electrolyte, the particles of the solid electrolyte are less likely to penetrate into the space between the particles of the cathode active material in the cathode active material layer. Therefore, compared to the secondary battery in which the liquid electrolyte is used in the electrolyte layer, the all-solid-state secondary battery is problematic in that the interface area between the cathode active material and the solid electrolyte is smaller and the ion conductivity is lower.
Accordingly, the cathode active material layer of the all-solid-state secondary battery is generally formed by using a mixture containing the cathode active material particles and the solid electrolyte particles, thereby securing the interface area between the cathode active material particles and the solid electrolyte particles and increasing the ion conductivity.